This invention relates to data communication systems in which messages travel sequentially through various stations of a network. One such network is the kind commonly referred to as a ring network. In a ring network a number of stations are connected by transmission links arranged in a ring and a message originating at any station is transmitted round the ring until it reaches its destination. The destination may be a single station, or more than one station may be a destination if the message is being broadcast to all of a given class of stations.
Many ring networks allow messages to pass right around the ring until they return to the originating station. In a number of such networks it has been proposed to exploit the fact that the message returns by including in it an acknowledgement field which is modified by a destination station, for example to indicate that the destination station has received and accepted the message. Thus the acknowledgement field may contain an "accepted" bit which is set by any station which accepts the message. However, with this procedure if the message is destined for more than one station the originating station will not be able to assure itself that every destination has accepted the message, merely that at least one has.